Before switching back to the Compose view, I have to ensure that Blogger is not going to quote my HTML so I hit Options on the right and make sure that Show HTML literally is selected.

Previewing the post, I can now see my Clojure code wonderfully highlighted as above.

This whole process is slightly faffy. What I really want is to remove the step where I have to find the right bit of HTML to copy into Blogger's raw HTML editor. One solution would be to figure out how to get htmlize to output HTML snippets (rather than whole documents). Another alternative would be to use org-mode - which works well with htmlize - to output the whole blog post. I'll write more if I get round to trying that.